Field of the Invention
This invention relates to an apparatus for producing a picture by dots and, more particularly, to an apparatus for producing a picture in which the reproduction of a density gradient is possible.
Description of the Prior Art
Various apparatuses which can reproduce a density gradient have been conventionally proposed. For convenience, the present invention will be described in detail hereinbelow using a an example an ink jet printer.
The following methods of reproducing a density gradient have been proposed conventionally.
A first method is to reproduce the density gradient by varying the dot diameter size to be printed by controlling the quantity of liquid discharged from the ink jet head.
As a second method, the dot diameter is not changed but one picture element (i.e. pixel) is constituted by a matrix consisting of, for example, micro-elements of 4.times.4 and the density gradient is reproduced using a dither process for this matrix.
However, according to the first method, it is difficult to take a wide range from the minimum dot diameter to the maximum dot diameter and thus only the reproduction of the order of several density gradients can be done. Thus, this method is unsatisfactory for printout of television pictures or photographs, or the like.
The second method eliminates the drawback in the first method, and if one pixel is constituted by the matrix of 4.times.4, seventeen density gradients can be reproduced. However, an increase of one pixel by 4.times.4=16 times larger than that in the first method causes the print speed to be reduced to 1/16, or the print speed must be made fast by increasing the number of print heads by 16 times. However, this countermeasure not only makes the construction of the print head complicated but also greatly increases the complexity of the electric circuit for processing pictures by means of the dither process, resulting in an extreme increase in overall cost.
To eliminate such prior art drawbacks, the present applicant has proposed an ink jet printer which uses a plurality of inks with different tinting densities and can cover a wide density range with a simple construction by controlling the dot diameter with respect to each ink in U.S. Ser. No. 491,654, filed on May 5, 1983.
On the other hand, in such an ink jet printer, the diameter of the print dot which can be formed by the ink jet head is 70-280 .mu.m when a certain type of head is used. To obtain a high density gradient by varying this dot size, the maximum dot diameter of about 200-280 .mu.m is needed. If the overlapped area is set to be small, a pixel is constituted by 4-6 dots/mm (dot/mm is referred to as PEL hereinbelow). While, for example, in a video printer which obtains reproduction pictures from television signals, the number of scanning lines of one frame of the television signal is 525 in case of NTSC system, so that the number of pixels is 525.times.(525.times.4/3) and the number of pixels in the effective screen among them is approximately 480.times.640 dots.
Therefore, when the screen is constituted by 5 PELs, the screen size becomes 96.times.128 mm, this size is suitable for viewing at an ordinary least distance of distinct vision.
When a plurality of tinting density inks are used, two methods are available to obtain the same reflection density: a method forming a small dot with an ink having a high tinting density; and a method forming a large dot with an ink having a low tinting density. However, even if reflection densities were identical, a large difference in the quality of such pictures would be apparent.
That is, when a change-over between high density dots and low density dots is made at a predetermined reflection density, the continuity of the picture is broken at the density change-over point. Therefore an unnatural picture would be produced.